1. Field of the Invention
This invention relates to a cathode ray tube apparatus and, more particularly, to a cathode ray tube apparatus having reduced deflection power consumption and a vacuum enclosure which has strong anti-atmospheric pressure strength.
2. Related Art
A conventional color cathode ray tube is shown in FIG. 8 and includes a vacuum enclosure which consists essentially of a glass panel 102 with an approximately rectangular-shaped display screen 101, a glass funnel 103 connected to the panel 102, and a cylindrical glass neck 104 connected to the funnel 103. A deflection yoke 109 is fixed at an outer peripheral section ranging in part from the neck 104 to the funnel 103. The funnel 103 includes the so-called yoke portion 110 which increasing in size from the junction with the neck 104 to the side of the funnel 103. The inner surface of the panel 102 is provided with phosphor screen 105 made of three-color phosphor layers of dot-like or stripe-like blue, green and red phosphors. A shadow mask 106 containing a large number of electron beam penetrating apertures is disposed opposite to the phosphor screen 105.
An electron gun 108 is provided to emit three electron beams 107 which are deflected in the horizontal and vertical directions by the horizontal and vertical magnetic fields generated by the deflection yoke 109, and scan, through the shadow mask 106, the phosphor screen 105 in the horizontal and vertical directions to display color images on the display screen 101.
A self-convergence, in-line type electron gun 108 has been widely used to emit the electron beams 107 in a row in a same horizontal plane, with the deflection yoke 109 set to generate a pin cushion type horizontal magnetic field and a barrel type vertical magnetic field so that the electron beams 107 are converged on the phosphor screen 105 over the entire panel 102 without any special correction components.
In a cathode ray tube of this sort, since the deflection yoke 107 is a high power consumption component, it is quite important to decrease power consumed in the deflection yoke 107 for the reduction of power consumption in the cathode ray tube. In practice, however, in order to increase screen brightness, a cathode voltage must eventually be raised to accelerate the electron beams. Further, the deflection frequency must also be high to comply with requirements for high definition TVs, personal computers, and other office automation equipment, resulting in increased power consumption in the deflection yoke.
Recently, strict restrictions have been imposed on leakage magnetic fields from the deflection yokes 109 of cathode ray tubes used for personal computers and other office automation equipment to which operators are always in close proximity. Generally, a compensation magnetic coil is conventionally added to reduce the leakage magnetic fields from the deflection yoke 109 but further increases the power consumption in the cathode ray tube.
It is desirable for a deflection yoke to apply its magnetic field to the electron beams efficiently. For that purpose, the neck of a cathode ray tube is made small in diameter, the deflection yoke fixed portion is made small in configuration, and a deflection magnetic field operating space is made small.
Since in the conventional cathode ray tube, however, the electron beams pass closely along the inner surface of the yoke portion, if the neck 104 is small in diameter and the yoke portion 110 is also made small in configuration, the electron beams emitted toward a diagonal edge of the phosphor screen 105 at the largest deflection angle as shown in FIG. 9(a) collide against the inner wall of the yoke portion 110 resulting in a "dead" zone 111 shown in FIG. 9(b). In the event that the electron 107 continue to collide with the inner wall, its temperature raises so high to melt the glass inner wall and, eventually, the cathode ray tube is in danger of implosion. It is, therefore, difficult to reduce the deflection power consumption by means of making the neck 104 or the yoke portion 110 small in size.
A countermeasure to solve this problem is described in Japanese Patent Application (Tokkohsho) No. 48-34349 corresponding to U.S. Pat. No. 3,731,129. Briefly, according to this reference, the yoke portion 110 is made rectangular in cross-sectional shape similar to the panel 102. When a rectangular raster is depicted on the phosphor screen, a region at the yoke portion where the electron beams pass through is also rectangular in cross-section. A cathode ray tube 113 shown in FIG. 10(a) has cross-sectional shapes taken along lines B--B through F--F shown in FIGS. 10(b) through 10(f), respectively. The shapes vary gradually from rectangular to circular. The panel 102 and the yoke portion 110 have approximately rectangular cross-sections but the neck 104 has semicircular and circular cross- sections. If the yoke portion 110 is made pyramid-like in configuration as shown, the long and short axes (the horizontal and vertical axes: H-axis and V-axis) of the deflection yoke are made small so that horizontal and vertical deflection coils of the deflection yoke are disposed close to the electron beams. With this structure, the electron beams are effectively deflected and the deflection power consumption, thus, can be reduced.
In such a cathode ray tube, as the cross-section of the yoke portion 110 becomes much closer to a rectangle, the horizontal and vertical axes 115 and 116 and the vicinities thereof are subject to more distortion in dotted-line directions 117 due to the atmospheric pressure F as shown in FIG. 11. The outer peripheries of the horizontal and vertical axes 115 and 116 of the yoke portion 110 receive compressive stress .sigma.H and .sigma.V while the outer periphery of the diagonal axis 118 and the vicinity thereof receive tensile stress .sigma.D. As a result, the vacuum enclosure has decreased anti-atmospheric strength and safety.
Further, it is desirable to make the panel as flat as possible in order to avoid outer light reflection thereon and to promote comfortable watching of images. Since that also decreases the strength of the vacuum enclosure, even if a conventional funnel with a pyramid-like deflection yoke portion is used, the strength necessary for safety cannot be always secured.
Because of the reasons set forth above, it is difficult to make the cross-section of the yoke portion rectangular to such extent as to produce sufficient reduction of the deflection power consumption, or, when a rectangular configuration is achieved, the strength of the vacuum enclosure is too weak to apply to a flat panel cathode ray tube.
The assignee of this application developed a technology concerning the pyramid-like shaped yoke portion as mentioned above and commercialized cathode ray tubes to which the technology was applied around 1970. In fact, two series of commercialized cathode ray tube apparatus were: 110.degree. deflection angle, 36.5 mm long neck diameter, and 18', 20', 22' and 26' diagonal lengths and 110.degree. deflection angle, 29.1 mm neck diameter, and 16' and 20' diagonal lengths. At that time, however, the cathode ray tubes had a spherical outer surface panel with an outer curvature radius of about 1.7 times the effective radius of the screen and were called 1R tubes (hereinafter also referred to as the "1R tubes"). The relationship between the configuration of the yoke portion and the vacuum enclosure strength was unknown in the case of cathode ray tubes having panel outer curvature radii 2 times or more the effective radius of the screen.
As set forth above, a reduction of deflection power consumption and a reduction in leakage magnetic field are nowadays required, but it is extremely difficult to comply with those requirements and, at the same time, to achieve high brightness, high frequency cathode ray tubes particularly used for high definition TVs, personal computers and other office automation equipment. Conventionally, it has been proposed that, as a structure to reduce the deflection power consumption, the pyramid-like configuration of the yoke portion varies from a circular cross-section at the neck to a rectangular cross-section at the funnel.
In the past, a cathode ray tube having a vacuum enclosure with sufficient strength as well as satisfactorily reduced deflection power consumption has not successfully been provided. This is particularly true in the case where the vacuum enclosure has a flat panel with an outer curvature radius which is twice the effective radius of its screen.